1. Field of the Invention
This invention relates to a drive device for driving a drill bit for drilling bore holes in underground formations.
2. Description of Prior Art
It is conventional to drive a drill bit by means of a rotary drive means disposed within a housing, the drill bit being attached to an output shaft driven by the rotary drive means. The housing is arranged to be connected to a drill string. The rotary drive means may be of the type which is powered by a working fluid, such as a working oil, which is forced through the housing under pressure, for example a turbine or Moineau-type motor.
The output shaft of such a drive means is subjected to axial forces in two opposite directions. Firstly, the pressure drop in the working fluid across the drive means generates a force in a downward direction and secondly, the reaction force of the drill bit in contact with the bore-hole bottom generates a force in an upward direction.
Ideally, these two forces are balanced but inevitably resultant axial forces will be generated when the forces are out of balance. Such conditions occur, for example, when factors other than balancing of axial forces are of prime consideration, for example a high drilling rate. Thus such devices require axial thrust bearings to absorb differences in axial forces over prolonged periods of operation.
German Patent Specification No. 2,250,415 discloses an axial bearing for a drilling turbine of the type described above in which a retaining ring carries a continuous bearing ring made of hardened steel and another retaining ring is provided with steel elements which are carried on elastomer supports located in blind holes disposed parallel to the axis of the shaft. The bearing ring and the steel elements have bearing surfaces which are in bearing contact with one another. The purpose of this arrangement is to dampen vibrations between the bearing surfaces and, in the case of a multi-stage design of bearing, to distribute the load between the individual stages. By arranging the steel elements in blind holes, an axial guidance is achieved which ensures that the bearing surfaces of the steel elements are constantly in full-area contact with the bearing surface of the bearing ring.
Such a bearing, however, involves direct metal to metal contact between the bearing surfaces of the steel elements and the bearing ring with the attendant problems as high wear rate.
An object of the present invention is to provide an improved axial bearing for use in a drive device for driving a drilling bit. A further object is to provide an axial bearing having improved wear characteristics.